The ability to measure and control the flow of fluids, both liquid and gas, is vital not only to research and development, but also to small and large scale production processes. Fortunately, the way in which a fluid stream divides between a large delivery channel and a small bypass channel can be well defined. A small, accurate flow rate transducer, applied to measure the flow in a small loop tapped off a large pipeline can be calibrated to reflect the flow rate in the pipeline; its electrical output signal can be used to control the fluid flow in the pipeline. Accurate transducers, therefore, that respond quickly to changing flow rates, provide stable, repeatable output signals and are of small size, are in demand for small and large applications alike.
One type flow rate transducer that can be made in a small package is the thermal mass flow transducer. It has long been known that the rate of heat transfer to a fluid in a laminar flow channel from the walls of the channel is a rather simple function of the temperature difference between the fluid and the channel, the specific heat of the fluid and the mass flow rate of the fluid within the channel. See, for example, P.B.S. Blackett, et al.; "A Flow Method for Comparing the Specific Heats of Gases"; Proc. R. Soc. London; A 126; pp. 319-354 (1930) (wherein the authors observed that where a laminar flow tube is provided with a constant gradient at zero flow, the nonlinear changes in the temperature profile of a tube to changes in the rate of gas flow through the tube is directly proportional to the product of the rate of flow and the specific heat of the gas flowing through the tube, p. 322) The thermal mass flow transducer is based on this relationship. Since the specific heat of a gas does not vary significantly with pressure or temperature, a thermal mass flow transducer calibrated for a particular gas can give true mass flow readings over a wide range of operating conditions.
Thermal mass flow transducers therefore include one or more heating elements for transferring heat energy to a fluid stream flowing in a small laminar flow tube, sometimes known as a capillary tube. The heating elements are usually made of an alloy having a high temperature coefficient of resistance. The tube is usually thin, and the elements are usually wound tightly around the outside of the tube to provide effective heat transfer to the fluid without disturbing the fluid flow within the tube. The high temperature coefficient makes these heating elements also very good devices for sensing the temperature of the tube, and they are often employed in that double capacity. For clarity, such double duty heating/sensing elements will be referred to herein as thermal elements.
Thermal fluid flow transducers have tended to develop into two basic varieties, which may be designated the differential sensing variety and the constant temperature variety. In the differential sensing variety of flow rate transducer, as disclosed by U.S. Pat. Nos. 3,851,526 and 4,548,075, for example, two identical thermal elements may surround a laminar flow tube in a symmetrical tandem arrangement, one element being upstream of the other. A constant current electrical source feeds both elements in a series circuit arrangement. The temperature differential between the elements is used as the measure of mass flow. The response of this transducer to a change in flow rate is relatively slow because of the need to reestablish equilibrium in the channel temperature profile for each reading.
In the constant temperature variety of flow rate transducer, as disclosed for example in U.S. Pat. No. 4,464,932, the laminar flow channel may be heated to a controlled temperature that is above the ambient. The power required to maintain the temperature of a single thermal element located within the temperature controlled area is used as the measure of fluid mass flow. Since the average temperature of the flow channel is held constant, this type transducer reacts much more quickly to flow rate changes than does the differential sensing variety, and it has met with considerable commercial success. In the known constant temperature flow transducers, however, the temperature profile of the flow channel does not in fact remain constant. As the flow rate increases, portions become cooler while other portions become hotter. Reestablishing thermal equilibrium involves the thermal inertia of the channel, and does take some time. Another disadvantage is that the output is not zero when there is no flow. It must be balanced by an offset voltage, introducing the problem of stability of readings.
A "hybrid" arrangement is described in pending U.S. Application No. 581,285 filed Sep. 12, 1990 in the name of Charles F. Mariano and assigned to the present assignee. This hybrid arrangement includes the principal advantages of both the differential sensing variety and the constant temperature variety of flow rate transducer without the disadvantages.
An object of the present invention is a flow rate transducer that responds quickly to flow rate changes.
Another object of the present invention is a constant temperature type flow rate transducer that is very stable.